1. Field of the Invention
This invention relates to methods of forming and of utilizing a compound billet and, more particularly, to methods of forming a compound billet, wherein a longitudinally extending inner member, composed of a first material, is retained within a hollow, longitudinally extending outer member, composed of a second and different material, and of thereafter deforming the compound billet so as to form an elongated, compound product.
2. Description of the Prior Art
It is known to form a compound billet, composed of two different materials, such as a copper-covered aluminum rod, and thereafter to form a compound product, e.g., by extrusion, from the compound billet. Two examples of such compound billets, and their formation, are provided by U.S. Pat. Nos. 3,620,059 (Nilsson) and 3,937,385 (Asada et al.). Compound billets of this general type are useful in forming copper-clad aluminum wires or busbars.
U.S. Pat. No. 3,620,059 (Nilsson) discloses a compound billet in which a longitudinally extending, copper outer member closely surrounds a longitudinally extending, aluminum inner member, the inner member including a groove in its periphery. A sealing member, such as an O-ring, prevents a pressure medium from penetrating between the outer and inner members during hydrostatic extrusion of the compound billet through an aperture in a die. Thus, the compressive effect of the pressure medium on the exterior of the outer member is to be utilized to hold the outer and inner members together during such extrusion. In addition, the outer member will initially be deformed, by the pressurization during extrusion or otherwise, so as to compress that portion of the outer member, surrounding the groove in the inner member, into the groove and, thereby, increase the adhesion between the outer and inner members.
U.S. Pat. No. 3,937,385 (Asada et al.) teaches forming a compound billet by first cleaning those surfaces of a longitudinally extending, aluminum inner member and a longitudinally extending, copper outer member which are to contact one another; then positioning the inner member within the outer member so as to bring such surfaces into contact and thereby provide a compound billet blank; next sealing the interior of the outer member under vacuum conditions; and then plastically deforming a short, longitudinally extending region on the exterior of the compound billet blank inwardly, e.g., by pinching and rotating such region between mating dies or blades, so as simultaneously to produce a conical shaped nose portion on the billet blank and to bond together the inner and outer members through plastic deformation at such region.
It has been determined, however, that the relaxation strain characteristic of copper, i.e., the partially restorative strain which is experienced upon the release of a stress of sufficient magnitude to plastically deform a body of copper, is greater than the relaxation strain characteristic of aluminum, i.e., the partially restorative strain which is experienced upon the release of a stress of sufficient magnitude to deform a similar body of aluminum. Thus, any application to the exterior of a copper-covered aluminum, compound billet blank of radially inwardly directed compressive stresses, i.e., compressive stresses which are directed radially toward a longitudinally extending axis of the blank, is unlikely to provide a firm grasp of the aluminum inner element by an inwardly deformed, copper outer element, since the outer element will tend to expand to a greater degree than will the inner element upon the release of such compressive stresses.
While the techniques of U.S. Pat. Nos. 3,620,059 and 3,937,385, as discussed, may provide useful compound billets for extrusion into elongated, compound products, it is believed that neither reliably provides a firm retention of a longitudinally extending inner member by a hollow, longitudinally extending outer member over a substantial portion of the area of engagement between the respective external and internal surfaces of the two members. Thus, some slippage of portions of the outer member with respect to adjacent portions of the inner member, e.g., along a longitudinal direction, may be present during extrusion, resulting in non-uniformities and/or other structural defects in the extrusion product. This is particularly true where the relaxation characteristic of the material of the outer member is greater than the relaxation characteristic of the material of the inner member, e.g., for a copper-covered aluminum billet, as is the case with respect to the two previously mentioned patents.